<p><b>Purpose:</b> The objective of the study was to estimate the presence of large mtDNA deletions in brain and spleen tissues of mice four months after exposure to 2 and 5 Gy.</p> <p><b>Materials and methods:</b> The male BALB/c mice underwent X-ray total-body acute radiation. Four months after irradiation, the mice were decapitated, and the samples of spleen and brain tissues were examined. A long-distance PCR was used to detect mtDNA deletions and their levels in samples of brain and spleen tissues.</p> <p><b>Results:</b> Four months after irradiation the levels of mtDNA deletions in the brain and spleen tissues were higher in animals exposed to 5 Gy than in animals at an irradiation dose of 2 Gy and in control mice. The levels of deletions in the mice brain tissues were higher 4 months than 1 month after X-ray exposure to both doses (2 and 5 Gy). In spleen tissues, a higher level of deletions was observed only at an irradiation dose of 5 Gy.</p> <p><b>Conclusions:</b> Our data have shown that ionizing radiation induces an increase of mtDNA copies with deletions in tissues of mice four months after the post-irradiation period. The level of deletions depends on the animal age, type of tissue, irradiation dose and length of the post-irradiation period.</p>